Delphi在TRichEdit中滚动居中的实现

Question

我有一个Delphi2007的TRichEdit，里面有几行代码。我想要垂直滚动richedit，使特定的行号大致居中于richedit的可见/显示区域。例如，我想在本例中为CenterLineInRichEdit编写代码：

procedure CenterLineInRichEdit(Edit: TRichEdit; LineNum: Integer);
begin
  ...
  Edit.ScrollTo(...);
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  REdit: TRichEdit;
  i: Integer;
begin
  REdit := TRichEdit.Create(Self);
  REdit.Parent := Self;
  Redit.ScrollBars := ssVertical;
  REdit.SetBounds(10, 10, 200, 150);
  for i := 1 to 25 do
    REdit.Lines.Add('This is line number ' + IntToStr(i));
  CenterLineInRichEdit(REdit, 13);
end;

我研究了使用WM_VSCROLL消息，它允许上下滚动一行，等等，但不能滚动到特定行的中心。

Answer 1

试一试；

procedure VertCenterLine(RichEdit: TRichEdit; LineNum: Integer);
// I don't know the reason but the RichEdit 2 control in VCL does not
// respond to the EM_SCROLLCARET in Richedit.h but it does so to the
// constant in WinUser.h
const
  EM_SCROLLCARET  = $00B7;
var
  TextPos: lResult;
  Pos: TSmallPoint;
begin
  TextPos := SendMessage(RichEdit.Handle, EM_LINEINDEX, LineNum, 0);

  if TextPos <> -1 then begin
    // Go to top
    SendMessage(RichEdit.Handle, EM_SETSEL, 0, 0);
    SendMessage(RichEdit.Handle, EM_SCROLLCARET, 0, 0);

    // Get the coordinates for the beginning of the line
    Longint(Pos) := SendMessage(RichEdit.Handle, EM_POSFROMCHAR, TextPos, 0);

    // Scroll from the top
    SendMessage(RichEdit.Handle, WM_VSCROLL,
        MakeWParam(SB_THUMBPOSITION, Pos.y - RichEdit.ClientHeight div 2), 0);

    // Optionally set the caret to the beginning of the line
    SendMessage(RichEdit.Handle, EM_SETSEL, TextPos, TextPos);
  end;
end;

下面的代码是另一种选择，因为它将字符串的第一次出现作为中心，而不是行号；

procedure VertCenterText(RichEdit: TRichEdit; Text: string);
const
  EM_SCROLLCARET  = $00B7;
var
  FindText: TFindText;
  TextPos: lResult;
  Pos: TSmallPoint;
begin
  FindText.chrg.cpMin := 0;
  FindText.chrg.cpMax := -1;
  FindText.lpstrText := PChar(Text);
  TextPos := SendMessage(RichEdit.Handle, EM_FINDTEXT,
      FR_DOWN or FR_WHOLEWORD, Longint(@FindText));

  if TextPos <> -1 then begin
    SendMessage(RichEdit.Handle, EM_SETSEL, 0, 0);
    SendMessage(RichEdit.Handle, EM_SCROLLCARET, 0, 0);

    Longint(Pos) := SendMessage(RichEdit.Handle, EM_POSFROMCHAR, TextPos, 0);
    SendMessage(RichEdit.Handle, WM_VSCROLL,
        MakeWParam(SB_THUMBPOSITION, Pos.y - RichEdit.ClientHeight div 2), 0);

    SendMessage(RichEdit.Handle, EM_SETSEL, TextPos, TextPos);
  end;
end;

Answer 2

向RichEdit发送EM_LINESCROLL消息：

SendMessage(REdit.Handle, EM_LINESCROLL, 0, NumberOfVerticalLinesToScroll);

请参阅EM_LINESCROLL MSDN topic。

Answer 3

基于这里的想法，我想出了一个解决方案。它假设richedit中的所有行都具有相同的高度，并且richedit的默认字体正确地报告了它的高度，但它可能对某些人有用：

type
  TCustomEditHack = class(TCustomEdit);

procedure CenterLineInEdit(Edit: TCustomEdit; LineNum: Integer);
var
  VisibleLines: Integer;
  TopLine: Integer;
  FirstLine: Integer;
begin
  FirstLine := Edit.Perform(EM_GETFIRSTVISIBLELINE, 0, 0);
  VisibleLines := Round(Edit.ClientHeight / Abs(TCustomEditHack(Edit).Font.Height));

  if VisibleLines <= 1 then
    TopLine := LineNum
  else
    TopLine := Max(LineNum - Round((VisibleLines/2)) + 1, 0);

  if FirstLine <> TopLine then
    Edit.Perform(EM_LINESCROLL, 0, TopLine - FirstLine);
end;

我用TRichEdit测试了这一点，但它可能也适用于TMemo。